Tuneable laser with improved suppression of auger recombination

ABSTRACT

A junction region for the laser diode may be improved to give an increased wavelength tuning range with improved thermal stability. The region has a homojunction structure that modifies the band structure to approximate that found in a type II superlattice. Up to half of the InGaAsP layer that nearest the p-InP region is n-type doped leaving the remainder with the original doping profile. This creates separate potential wells for electrons and holes in different parts of the InGaAsP layer. Also the barrier for electrons, but not for the holes, on the (p-InP)-(I-InGaAsP)-heterojunction may be increased by inserting a blocking layer of InAlAs, which is lattice matched to InP and InGaAsP, on the p-side between the above two materials.

The present invention relates to a tuneable laser. More particularly,but not exclusively, it relates to a laser junction region having anincreased electron barrier and/or a reduced Auger recombination rate.

Narrow band lasers are important for a number of applications in opticaltelecommunications and signal processing applications. These includemultiple channel optical telecommunications networks using wavelengthdivision multiplexing (WDM). Such networks can provide advancedfeatures, such as wavelength routing, wavelength conversion, adding anddropping of channels and wavelength manipulation in much the same way asin time slot manipulation in time division multiplexed systems. Many ofthese systems operate in the C-band in the range 1530 to 1570 nm.

Tuneable lasers for use in such optical communications systems,particularly in connection with the WDM telecommunication systems, areknown. A known tuneable system comprises stacks of individuallywavelength distributed Bragg reflectors (DBR) lasers, which can beindividually selected, or by a wide tuning range tuneable laser that canbe electronically driven to provide the wavelength required. Due to thecrucial importance of the tuning process for the application of laserdiodes in optical telecommunications and optical interconnects there isactive and sustained interest in developing new tuning mechanisms andoptimising existing ones. The invention in this patent addresses thelatter point.

In monolithic semiconductor diode lasers such wavelength tuning can beachieved by a number of methods that utilise different physicalproperties of the materials used in the construction of the lasers.

Wavelength tuning may be accomplished in several ways.

Firstly, there is the free carrier plasma effect, in which theelectronic dielectric function dispersion ε(ω) is dependent on the freecarrier density no and this is used to modify the optical properties ofthe medium${{ɛ(\omega)} = {ɛ_{\infty}\left( {1 - \frac{\omega_{p}^{2}}{\omega^{2}}} \right)}},$Where$\omega_{p} = \left( \frac{n_{0}e^{2}}{ɛ_{0}ɛ_{\infty}m^{*}} \right)^{\frac{1}{2}}$is a plasma frequency.

-   ε_(∞) is the high-frequency lattice dielectric constant-   m^(*) is the electron effective mass-   ε₀ is the vacuum permittivity

A change in the electron density n, from the injection of free carrierscauses a change in the plasma frequency q. This leads to a change in therefractive index n (ω) of the material since n² (ω)=ε(ω). An increase inthe electron density results in a decrease in the refractive index ofthe material.

The advantages of this tuning mechanism are it's relatively high tuningspeed, up to approximately 1 GHz, the large wavelength tuning range andthe ability to realise continuous tuning. The drawback of the mechanismis that the injected electron-hole pairs subsequently recombine and thisrequires a sustained current that leads to heat generation in thedevice.

Secondly, there is the quantum confined Stark effect (QCS), which may beutilised with quantum well structures. The Franz-Keldysh effect isexploited in the multiple quantum well heterostructure with the electricfield applied normal to the quantum well interfaces. The electric fieldinduces a change in the energy differences between the electron and holeground states in the quantum well and also displaces the centres of theelectron and hole wavefunctions with respect to each other. As aconsequence the electron-hole transition matrix element is reduced andthe electronic refractive index changes.

The refractive index change due to the QCS effect is negative, similarto the change caused by any free electron plasma effect. It should benoted that unlike bulk materials, the interaction of light with chargecarriers near the bandgap is primarily due to excitonic effects ratherthan free carriers in the quantum well structures.

At wavelengths close to excitonic resonance the refractive index changesin the quantum well heterostructure are two orders of magnitude largerthan in bulk material. In III-V semiconductors the refractive indexchange is typically of the order of 10⁻³ to 10⁻².

The advantages of this tuning mechanism are firstly the high tuningspeed, there are practically no internal time constants and the speed islimited only by external parasitic elements and secondly there isnegligible heat generation. However the tuning range realised by thisscheme is considerably smaller than that achieved with the free electronplasma effect and the effect is temperature sensitive.

However, practical realisation of the scheme is technically demandingsince the maximum change in the refractive index takes place atwavelengths close to the exciton resonance where absorption is alsolarge.

Finally, there is thermal tuning, in which the bandgap of a material andits Fermi distribution parameters depend on the ambient temperature.Consequently temperature can be used as a means to vary the emissionwavelength and refractive index of the laser medium. A point to note isthat unlike the previous two effects, where the changes in therefractive index were negative, increasing the temperature will decreasethe bandgap and increase the refractive index.

The advantages of the thermal tuning scheme are the relative simplicityand relatively large tuning range. The disadvantages are the very largeheat generation in the devices and the very low tuning speed. Tuningrequires either the heating or cooling of the laser chip and suchprocesses do have large time constants.

Of the three mechanisms described above the free electron plasma effectis most commonly used in monolithic, continuously tuneable semiconductorlaser diodes. In order to tune the emission wavelength, free carriersare injected via electrodes into the tuning region of the laser. Asstated earlier increasing the electron density will reduce therefractive index of the material

In general, two effects limit the maximum tuning range:

-   (i) The rise in the device temperature due to current heating    results in a positive shift in wavelength that acts in opposition to    the shift caused by the free carrier plasma effect.-   (ii) At high hole densities the optical losses due to inter-valence    band absorption increase, thus the optical output power decreases    with increasing tuning current.

Therefore the optimal operation regime of the tuning section of thelaser will occur when the maximum free electron density n_(o) isachieved for a minimum injection current I.

The tuning efficiency due to carrier injection decreases at high carrierdensities because of non-linear recombination mechanisms. The mainrecombination process is Auger recombination.

The tuning current is given by:I=eV(C ₁ n _(o) ² p ₀ +C ₂n_(o) p _(o) ²)

-   -   where:        -   n_(o) is the electron density        -   p_(o) is the hole density        -   C₁ & C₂ are the Auger recombination coefficients        -   V is the volume of the tuning region

In the normal operation of the laser diode there is a high injectioncurrent and low doping in the tuning region of the laser and thus thefollowing approximations can be made n_(o)≈p_(o) and so I=eVCn_(o) ³.

From the above expression it can be seen that for a given electron andhole densities n_(o) & p_(o) the minimum current can be achieved byeither (a) decreasing the volume V of the tuning section, or (b)decreasing the Auger coefficients.

The volume V of the tuning section is however predetermined by thedesign of the tuning section, normally the size of the sampled gratingdesign, and by the dimensions of the laser's active gain section.

The Auger recombination coefficient is a material property. It could bedecreased in theory by choosing a material for the tuning section with alarge bandgap energy or one which has indirect conduction—valence bands.However integration of the active gain and tuning section on the samelaser chip impose design constraints on the choice of materials whichmake this unfeasible.

The solution is to suppress the Auger recombination rate by creatinginhomogeneous electron and hole distribution profiles n({right arrowover (r)}) and p({right arrow over (r)}) in the tuning region in such amanner that the electron and hole densities remain high but the overlapbetween the electron and hole profiles is small i.e. the productsn²({right arrow over (r)})p({right arrow over (r)}) and n({right arrowover (r)})p²({right arrow over (r)}) are minimal in the tuning region.

It is known in the art that the incorporation of a type II superlatticeinto the tuning region is a means to decrease the effective Augerrecombination rate. In such a heterostructure the electrons and holesare spatially separated as shown in FIG. 1. In the type UI superlatticesthe sign of the energy and discontinuity at each interface is the samefor the conduction bands and for the valence bands. As a result of thisin each separate layer there exists a potential quantum well forelectrons (holes) and a potential barrier for holes (electrons). Thesituation is different in type I superlattices where in one layer thereis a quantum well for electrons and holes while in an adjacent layerthere is a barrier for both types of carriers.

In FIG. 1 the bandgap energy of materials in layers 1 and 2 is chosen tobe constant. The electrons are confined in layer 1, while the holes areconfined in layer 2. This separation, of course, is not complete sincedue to finite height of the barriers the electron and hole wavefunctions penetrate into the adjacent barriers. Also, due to thermalexcitation there is a number of electrons above the barrier in layers IIand there is a number of holes above the barrier in layers LNevertheless, it is assumed that the majority of electrons will be inlayers I and majority of the holes will be in layers II. As a result ofthis the products of the electron and hole densities n_(I(II)) andp_(I(II)) in each layer are small: n_(I(II)) ²p_(I(II))<<n₀ ³ andn_(I(II))p_(I(II)) ²<<n₀ ³. This will suppress an average Augerrecombination rate and reduce the current consumption in the tuningregion with incorporated superlattice in comparison with the case ofbulk tuning region.

Semiconductor materials for the tuning region with a bandgap wavelengthof 1.3 μm being lattice matched to InP have been studied by S. Neber andM-C Amann, “Tuneable laser diodes with type II superlattice in thetuning region”, Semicond. Sci. Technol. 13 801-805 (1998). Allquaternary combinations of Al, Ga, In, As, Sb, and P were taken intoaccount. As a result InGaAsP, AlGaInAs, and AlGaAsSb were identified assuitable semiconductors. The results for these materials are given inTable 1 and Table 2: TABLE 1 Δa/a Ec E_(v) ^(lh) E_(v) ^(hh) Eg Material(%) x y (eV) (eV) (eV) (eV) InP 0 −5.649 −7.003 1.354 Al_(x)In_(1−x)As 00.475 −5.465 −6.880 1.415 Ga_(x)In_(1−x)P_(y)As_(1−y) 0 0.328 0.202−5.808 6.758 0.950 Ga_(x)In_(1−x)P_(y)As_(1−y) 1, compressive 0.0510.576 −5.845 −6.851 −6.795 0.950 Ga_(x)In_(1−x)P_(y)As_(1−y) 1, tensile0.546 11,215 −5.768 −6.718 −6.794 0.950 Al_(x)Ga_(1−x)As_(y)Sb_(1−y) 00.075 0.516 −5.506 −6.456 0.950 Al_(x)Ga_(1−x)As_(y)Sb_(1−y) 0 0.1190.519 −5.443 −6.476 1.033 Al_(x)Ga_(1−x)In_(1−x−y)As 0 0.151 0.310−5.784 −6.734 0.950

TABLE 2 Material 1 GaInPAs GaInPAs AlGaAsSb Material 2 AlGaAsSb AlGaInAsAlGaInAs Band off set (meV) 302 24 278

The combination GaInAsP/AlGaAsSb is most suitable for the necessaryrealisation as it offers the largest band offsets. The obtained resultis shown in FIG. 2 where the mean electron density as a function oftuning current for a type II superlattice with ΔE=302 meV, a constantbandgap wavelength of 1.3 μm and equal layer thicknesses d₁=d₂.

For comparison the case of the bulk tuning region is also shown in FIG.2. In order to obtain a consistent result at a small current, theequation for current has been modified by including the terms whichdescribe the contribution of the Shockley-Read-Hall recombination (∝An)and radiative band to band recombination (∝Bn²). The mean electrondensity was obtained by integration over one period of the superlatticeand dividing by (d₁+d₂). The mean electron density and the mean holedensity are assumed to be equal in order to maintain overall chargeneutrality.

The calculations showed significant increases in mean electron density.Even at high tuning current the mean electron density is enhanced by afactor of about 3 in comparison with the bulk tuning region. At smallcurrents the improvement factor is about 150.

However, in practise the improvements would not be as significant, sinceall superlattice layers cannot be considered as bulk materials withdifferent properties. The thickness of the tuning section is in realityabout 300-400 nm and an incorporation of 5-6 periods of the superlatticewill result in the thickness of each layer being about 30 nm.

In this case the layers will be well connected with each other due toquantum overlap of the wavefunctions in neighbouring layers (especiallytaking into account that the barriers are not very high, only about 0.3eV). Also, due to the 2D quantisation of the energy levels in eachlayer, the effective barrier height will be smaller than the bandsoffset. Therefore, a bulk-like interpretation of the layers mayoverestimate the spatial separation of the electrons and holes.

Also the conduction and valence band profiles as shown in FIG. 1 containa flat band approximation. In reality the bands will be bent which willdecrease the actual barrier height.

Furthermore an external electric field will modify the band profiles.The tuning section is in fact a p-i-n diode and the external bias willdrop mainly in the undoped i-region. In this case the potential profileshown in FIG. 1 will be transformed.

A schematic profile in the presence of electric field is shown in FIG.3, from which it can be seen that, due to conduction and valence bandinclination, the electrons and the holes in adjacent layers come closertogether than in the case of zero electric field, as indicated by thedashed oval line 14 in FIG. 3. This will increase the recombination rateand thus will decrease the average electron density in the tuningsection.

Incorporation of type II superlattice is essentially equivalent to thecreation of an artificial semiconductor with indirect conduction andvalence bands and the materials forming the superlattice may well not beideally matched to the other materials used in the construction of thelaser. As stated above, the GaInAsP/AlGaAsSb combination theoreticallygives the largest band offset and hence reduction in Augerrecombination. However such a material system is not necessarilycompatible with a number of material systems used in the manufacture oflaser diodes.

Another factor that needs to be considered is the location at whichcarrier recombination actually occurs. To achieve maximum tuning the aimis to achieve maximum carrier density in the tuning section for a giveninjection current. As stated above this may be done by suppressing theAuger recombination in the tuning section, assuming that all theinjected carriers, both electrons and holes, will subsequently recombinethere.

However consideration of the tuning section of a InP/InGaAsP/InP p-i-ndiode laser, as shown in FIG. 4, shows that there are device structureconstraints on the efficiency of such recombinations. FIG. 4 shows theband structure of the tuning region of such a laser with an IaP p-typeregion 20, an InGaAsP intrinsic un-doped region 21 and an InP n-typeregion 22. At either side of the section are ohmic contacts 23 and 24.Marked are the positions of the conduction band E_(C) and the valenceband E_(V). Also shown is the difference in energy in the conductionband between the intrinsic un-doped region and the doped regions, ΔE_(C)and the bandgaps for the InP and InGaAsP regions marked as 25 and 26respectively.

FIG. 5 shows a schematic potential energy profile of the tuning regionof a laser diode that incorporates the effect of band-bending.Band-bending occurs due to the requirement that the quasi-Fermi levelshould be constant throughout the whole of the structure. As can be seenfrom FIG. 5 the barrier height seen by electrons in the i-region,ΔE_(C)*, is increased compared to ΔE_(C) when the effects of bandbending is accounted for.

Only carriers injected into the undoped i-region 21 will contribute tothe change in refractive index and not all of the injected carriers willrecombine in the undoped region. This is because the leakage current maytake a considerable fraction of the injected carriers away from thetuning region. The total leakage current is the sum of the electroncurrent in the p-region 20 and the hole current in the n-region 22 ofthe p-i-n heterostructure. As a result of this leakage current theeffective number of carriers available for recombination in the i-regionis decreased resulting in a lower tuning efficiency.

The leakage current is relatively large because the heterojunctionbarrier height, ΔE_(C), for electrons is relatively small. Assuming awavelength of λ=1.42 μm and that the ratio for the conductionband/valence band offset is 40/60 then the resulting band gaps are E_(g)^(InP)=1.35 eV (25) and E_(g) ^(InGaAsP)=0.873 eV (26). This givesΔE_(C)=0.191 eV and ΔE_(v)=0.286 eV.

Another factor to consider is the effect of the small electron effectivemass in the InGaAsP. In this region m_(n) ^(*)=0.05 m₀ and this resultsin a small density of states in the i-region. Thus for typical injectedelectron densities n₀≈2-3×10¹⁸ cm⁻³ the Fermi energy E_(F)=0.15 eV. Thusthe Fermi energy Ep of the injected electron gas in the i-region iscomparable to the above barrier height resulting in an effective barrierheight of less than 50 meV. This will result in a relatively largeelectron leakage current. The hole leakage current will be considerablysmaller due to the hole effective mass being an order of magnitudelarger than the electron effective mass which results in a smaller Fermienergy and the higher potential barrier for holes.

Thus the maximum tuning efficiency will be achieved by decreasing theelectron leakage current over the heterojunction from the i-region andsuppressing the Auger recombination in the i-region. The presentinvention addresses these problems and provides a device structure thatprovides a solution to carrier localisation and reduction innon-radiative recombinations.

It is an object of the present invention to provide a heterojunctionstructure that provides firstly a means for the spatial localisation ofthe different carrier types and hence a reduction of non-radiative Augerrecombination, and secondly a means of electron leakage currentreduction.

According to a first aspect of the present invention, there is providedthe tuning section of a tuneable laser incorporating a novelhomojunction structure that modifies the band structure to approximatethat found in a type II superlattice.

The tuning section of the tuneable laser may be a(p-InP)-(i-InGaAsP)-(n-InP) structure, wherein up to half of the InGaAsPlayer is doped leaving the remainder in the original intrinsic state, soas to create separate potential wells for electrons and holes indifferent parts of the InGaAsP layer.

Preferably the region of the i-InGaAsP nearest the p-InP is n-typedoped, with the remainder of the region being undoped. In this casethere is a potential well for electrons in the n-doped region of thInGaAsP layer and a potential well for holes in the undoped region ofthis layer.

Alternatively the region of the i-InGaAsP nearest the p-InP is n-typedoped, with the remainder of the region being p-doped. In this casethere is an enhanced potential well for electrons in the n-doped regionof the InGaAsP layer and an enhanced potential well for holes in theundoped region of this layer.

According to a second aspect of the invention, there is provided thetuning section of a tuneable laser incorporating a hetrojuctionstructure comprising a blocking layer between the two materials thereofon the pAside so as to increase the barrier for electrons only, but notfor holes, while maintaining the same injection level for the electronand hole current.

The material of the blocking layer has to be latticed matched to theadjacent layers. The blocking material may be InAlAs or InAlAsP for thecase of a tuning section of the tuneable laser having a(p-InP)-(i-InGaAsP)-(n-lnP) structure.

Preferably, insertion of the blocking layer provides an additionalbarrier for the electrons of about 0.2 eV and substantially noadditional barrier for the holes.

According to a third aspect of the present invention, there is provideda tuneable laser comprising in combination any feature of the firstaspect described above and any feature of the second aspect describedabove.

Embodiments of the present invention will now be more particularlydescribed by way of example and with reference to the accompanyingdrawings, in which:

FIG. 1 shows schematically a known tuning region including a type IIsuperlattice giving a heterostructure in which electrons and holes arespatially separated;

FIG. 2 shows graphically the mean electron density as a function oftuning current for a type II super lattice with ΔB=302 meV;

FIG. 3 is a schematic profile of the prior art tuning region in thepresence of electric field;

FIG. 4 shows the tuning section of a known InP/InGaAsP/InP p-i-n diodelaser;

FIG. 5 shows a schematic potential energy profile of the tuning regionof a laser diode incorporating the effect of band-bending;

FIG. 6 shows the resulting band structure when the region of thei-InGaAsP nearest the p-InP region is n-type doped, leaving theremainder undoped;

FIG. 7 shows schematically a structure having a p-In region, an n-dopedInGaAsP region, an intrinsically doped InGaAsP (i-InGaAsP) region, andan n-doped InP region;

FIG. 8 shows a two terminal p-n-p-n Shockley diode;

FIG. 9 shows a schematic potential energy profile as shown in FIG. 5,with an additional Fermi level shift;

FIG. 10 shows the energy band profile of the structure beforeincorporation of a blocking layer;

FIG. 11 shows the energy band profile of the structure of FIG. 10 whenan InAlAs blocking layer is inserted;

FIG. 12 shows the resulting band structure when the two aspects of theinvention are combined;

FIG. 13 shows the band structure of FIG. 12 with the remaining part ofthe i-region being p-doped;

FIG. 14 shows the physical layer structure of a tuning region of a laserdiode which incorporated delta doping; and

FIG. 15 shows the band structure of a tuning region of a laser diodewhich incorporated delta doping.

To achieve maximum tuning at minimum possible injection current it isnecessary to reduce the amount of Auger recombination that takes placein the tuning region. It is known that the only method of reducing theAuger recombination rate that is technically achievable within a tuningsection where there are constraints on the material systems used is thephysical separation of the electrons and holes. This may be accomplishedby modulating the doping profile within the i-region to separatespatially the carriers, with the resulting structure approximating thatof a type II superlattice, resulting in an enhanced carrier density inthe i-region.

Separation between electrons and holes in the i-InGaAsP layer may beachieved by the use of u-type doping within this layer. Up to half ofthe InGaAsP layer is doped leaving the remainder with the originaldoping profile which creates separate potential wells for electrons andholes in different parts of the InGaAsP layer. To achieve the requiredseparation, the region of the i-InGaAsP nearest the p-InP region isn-type doped, such doping being in the range 10¹⁷-10¹⁸ is cm⁻³, leavingthe remainder un-doped. FIG. 6 shows the resulting band structure,taking account of band bending.

The resulting structure is a p⁺-n-i-n⁺ system. FIG. 7 showsschematically the structure where region 51 is p-doped InP, region 52 isn-doped InGaAsP, region 53 is intrinsic InGaAsP (i-InGaAsP) and region54 is n-doped InP. The junction between the p-InP and the n-dopedInGaAsP is 55, the junction between the n-doped InGaAsP and thei-InGaAsP is 56 and the junction between the i-InGaAsP and the n-IaP is57. The result will be even more pronounced if the remaining part of thei-region 53 is p-doped, such doping being in the range 10¹⁵-10¹⁷ cm⁻³.However the doping has to be moderate to avoid increases in the opticallosses mentioned above. This system is formally similar to a twoterminal p-n-p-n Shockley diode, as shown in FIG. 8.

The structure embodied in the present invention is a Shockleyheterodiode since there is an external p⁺-n heterojunction on the leftside of the structure and an external i-n⁺ heterojunction on the rightside of the structure. The centre n-i (or n-p) junction is ahomojunction.

It is well known for a Shockley diode that for a positive anode andnegative cathode voltage in the forward blocking regime the two externaljunctions are forward-biased and operate as effective emitters forelectrons and holes, respectively, while the centre junction isreverse-biased. The electric field at the centre junction tries toseparate the injected electrons and holes. Consequently the resultingcurrent flowing through the structure remains very small and at the sametime the density of injected electrons and holes is high.

FIG. 6 shows the energy band profile at thermodynamic equilibrium forthe structure proposed. Shown are the two main effects of the modulationof the doping profile in the i-InGaAsP:

-   -   (a) The actual barrier for electrons at the        (p-InP)-(n-InGaAsP)-heterojunction increases; and    -   (b) There is a potential well 41 for electrons in the n-doped        region of the InGaAsP layer and a potential well 42 for holes in        the undoped region of this layer.

The injected electrons will occupy the potential well 41 as shown by 43and the injected holes will occupy the potential well 42 as shown by 44.This results in spatial separation of the electrons and holes. Thecarriers are not completely separated due to the relatively shallownature of the potential wells 41 and 42.

When an external direct bias is applied during forward blocking, thereverse bias of the centre junction 56 is increased and the depth of thepotential wells and the electric field strength at the junction will bealso increased. This will lead to additional separation of the electronsand holes in InGaAsP layer and to suppression of the Auger recombinationin this layer.

The electron leakage current over the heterojunction from the i-regionis approximately an exponential function of the heterobarrier height. Inorder to decrease the leakage current it is necessary to increase thepotential barrier for the electrons.

A further increase in barrier height maybe obtained by increasing thedoping level in the p-region then the new Fermi level E_(Fp) in thep-region will lie closer to the valence band edge than in the case shownin FIG. 5. There is an additional Fermi level shift ΔE_(F), as it shownin FIG. 9. As a result of this shift the new energy band profile willhave an additional difference between the band energy in the p-regionand in the i-region. The actual barrier height which is seen by theelectrons on the i-p-heterobarrier is approximately equalΔE_(C)≈ΔE_(C)*+ΔE_(F). In principle, this means that at very high levelof p-doping the heterobarrier height may be close to the bandgapdifference between E_(g) ^(InP) and E_(g) ^(InGaAsP) instead of theconduction bands offset.

However such an increase in the p-doping level will result in increaseof the optical losses due to the hole intervalence band absorption. Thisproblem can be avoided if the high level doping profile will stop somedistance away from the p-i-heterojunction.

Another way to increase the electron and the hole separation is to usethe delta-doping. Two adjacent delta-doped layers, n-type doped layer onthe left side and the p-type doped layer on the right side of thecentral junction, as shown in FIG. 14, will modify the potential barriershape and create an additional built-in electric field located at thecentre of the junction due to additional Fermi level alignment in thedelta-doped regions, as shown in FIG. 15. The built-in field has suchdirection that it will push the electrons and holes in oppositedirections further away from the junction

An alternative method for increasing the barrier for electrons on the(p-InP)-(i-InGaAsP)-heterojunction is to insert another material (ablocking layer) on the p-side between the above two materials. Theappropriate candidate should increase the barrier for the electronsonly, but not for the holes, in order to maintain the same injectionlevel for the hole current This material should also be lattice matchedto InP and InGaAsP.

Inspection of Table 1 shows that an appropriate material could be forexample, InAlAs. It also may be InAlAsP as well. FIG. 10 shows theenergy band profile of the structure before incorporation of theblocking layer, and FIG. 11 corresponds to the case when the InAlAslayer is inserted. The two diagrams have not included the effects ofband bending for simplicity. It can be seen that insertion of theblocking layer provides an additional barrier for the electrons of about0.2 eV and there would be no additional barrier for the holes. This willhave the effect of decreasing of the electron leakage current but itwill not effect the hole injection current.

Combining the two aspects of our invention achieves maximum tuningefficiency. The introduction of an increased barrier height willdecrease the electron leakage current over the heterojunction from thei-region and thus increase the carrier density in the i-region. Themodulation of the bandgap in the i-region will reduce the non-radiativerecombinations by suppressing the Auger recombinations.

FIG. 12 shows the resulting band structure when the two aspects of ourinvention are combined. FIG. 13 shows the resulting band structure whena section of i-region is p-doped to enhance the separation of thecarriers.

A device embodying one or more aspects of the invention may giveincreased wavelength tuning range. Due to the reduction of the tuningcurrent, there will be a considerable decrease in heating of the device,which in turn will improve thermal stability and increase the tuningspeed of the laser diode. The reduced tuning current also leads to lessfree carrier absorption, which in turn leads to a decrease in the outputpower non-uniformity.

1. A tuneable laser having a timing section comprising a homojunctionstructure that modifies the band structure to approximate that found ina type IT superlattice.
 2. A tuneable laser having a tuning sectioncomprising a homojunction structure comprising p and i layers, whereinup to half of the i layer is doped leaving the remainder with theoriginal intrinsic state, so as to create separate potential wells forelectrons and holes in different parts of the i layer.
 3. The tuneablelaser as claimed claim 2, wherein the region of the i layer nearest thep layer region is n-type doped, leaving the remainder undoped, to createa potential well for electrons in the n doped region of the i layer anda potential well for holes in the undoped region of this layer.
 4. Atuneable laser having a tuning section comprises p and i layers, whereinthe region of the i layer nearest the p layer is n˜type doped, with theremainder of the region being p-doped, so as to create separatepotential wells for electrons and holes in different parts of the ilayers.
 5. The tuneable laser according to claim 2, wherein the p layeris of InP.
 6. The tuneable laser according to claim 2, wherein the ilayer is InGaAsP.
 7. The tuneable laser according to claim 2, whereinthe homojunction structure modifies the band structure to approximatethat found in a type n superlattice.
 8. A tuneable laser having a tuningsection including a heterojunction comprising a blocking layer betweenthe two materials thereof on the p-side, so as to increase the barrierfor the electrons only, but not for the holes, while maintaining thesame injection level for the electron and hole current.
 9. The tuneablelaser as claimed in claim 8, wherein insertion of the blocking layerprovides an additional barrier for the electrons of about 0.2 eV andsubstantially no additional barrier for the holes.
 10. The tuneablelaser as claimed in claim 8, wherein the heterojunction is a(p-InP)-(i-InGaAsP)-structure_(>) and the material of the blocking layeris lattice matched thereto.
 11. The tuneable laser as claimed in claim10, wherein the blocking layer comprises InAlAs.
 12. The tuneable laseras claimed in claim 10, wherein the blocking layer comprises InAlAsR 13.The tuneable laser as claimed in claim 8, further comprising twodelta-doped layers, one on each side of and adjacent to a centraljunction,
 14. (cancelled)